Light-Driven Abiotic Formation of Dimethyl SelenylSulfide in the Liquid and Gas Phases

Biogenically produced volatile organic selenium (Se) species such as dimethyl selenyl sulfide (CH₃SeSCH₃) and dimethyl diselenide (CH₃SeSeCH₃) are important sources of Se in the atmosphere. Once emitted, Se can travel through the atmosphere and subsequently deposit to soils, impacting available Se in food crops. To improve the predictive capabilities of the sources and sinks of CH₃SeSCH₃, we studied its abiotic light-driven formation and subsequent photochemical decay. Upon UVA irradiation of CH₃SeSeCH₃ and its more abundant sulfur analogue, dimethyl disulfide (CH₃SSCH₃), we observed the formation of CH₃SeSCH₃ in both the liquid and gas phases. We unambiguously confirmed the synthesis of CH₃SeSCH₃ by GC-MS and ¹H, ¹³C, and ⁷⁷Se NMR spectroscopy before studying this process by online Vocus mass spectrometry in the gas phase. The photolysis by UVA light produced selenyl and thiyl radicals, which formed CH₃SeSCH₃ in 36% yield. Moreover, we identified photo-oxidation products and subsequent SOA formation. Our mechanistic analysis concludes that the detection of CH₃SeSCH₃ in the environment coexists with CH₃SeSeCH₃ and CH₃SSCH₃. Thus, CH₃SeSCH₃ in the environment, for example, above a phytoplankton bloom, may have a combination of both primary and secondary sources. Our study shines light on CH₃SeSCH₃’s synthesis, isolation, and environmental transformations.